First, it is not clear to me that angels know how to sing. Many Christians assume they do, but I'm not so sure.
Notice the text does not say the angels sang; rather, they praised God and said. Several Christmas carols refer to singing angels, such as
"Angels we have heard on high, sweetly singing o'er the plains, And the mountains in reply echo back their joyous strains."
"Hark, the herald angels sing, Glory to the newborn king."
Again, however, the Bible does not clearly and unequivocally state that angels do, in fact, sing.
". . . the four living creatures and the twenty-four eldersthe four living creatures and the twenty-four elders fell down before the Lamb. Each one had a harp and they were holding golden bowls full of incense, which are the prayers of the saints. And they sang a new songsang a new song: 'You are worthy to take the scroll and to open its seals, because you were slain, and with your blood you purchased men for God from every tribe and language and people and nation. You have made them to be a kingdom and priests to serve our God, and they will reign on the earth'" (vv.8-10, my emphasis).
Obviously, there will be singing in heaven (and mayalmost certainly already beis), but the angels evidently do not sing. Later in Revelation chapter 5, we read,
In other words, only those who have been redeemed by the blood of the Lamb are ready, willing, and able to sing, since they, unlike the angels, have been forgiven of all their sins. The holy angels which will join (and probablymost certainly have already have joined) the celebration in heaven one day, but only redeemed men and woman, boys and girls, will have a ready song on their lips for all eternity.